A common loudspeaker system includes a single diaphragm, moving coil and has a simple construction and is quite dependable. It is a fundamentally correct design as its sound source essentially collapses toward its center as the frequency increases. As such, it is a practical embodiment of the theoretically ideal “point source” transducer, and if well designed, it can exhibit a facsimile of the input signal, albeit over a limited bandwidth of frequencies usually near the middle of its range.
The audible sound spectrum comprising approximately ten octaves (a doubling or halving of frequency) from 20 Hz-20 kHz has proven virtually impossible for any single diaphragm, which is mass-controlled, to replicate accurately. One reason for this is because the requirements for propagating low frequencies (long wavelengths) and high frequencies (short wavelengths) are very different, and therefore mutually exclusive.
In light of the need to replicate or cover the full frequency range more accurately, specially designed transducers have been developed which cover overlapping bands of frequencies. Some of these specially designed transducers are connected to a frequency dividing network or crossover, either passive, electronic or both, which functions to divide the frequencies of the output of an audio amplifier or amplifiers into frequency bands which are directed to the respective transducers constructed to reproduce those bands.
Crossover frequencies are primarily determined by the usable bandwidth of the transducer. For tweeters (high frequency transducers), it is usually determined by the resonant frequency and the crossover point should be one or, more preferably, two octaves above the resonance frequency. The upper limit for woofers (low frequency transducers) is usually determined by the horizontal polar response. As the frequency increases, and the wavelength becomes the same size or smaller than the diameter of the transducer, the diaphragm's acoustic output becomes restricted to progressively narrower solid angles, and begins to become very directional. The most often used criterion for the crossover point of a woofer is the frequency at which the output is six decibels down (−6 db) at forty-five degrees (45°) off axis.
These two-way loudspeaker systems (the “two” ways implying the presence of two transducers such as a woofer and a tweeter) generally exhibit wider dispersion of the higher frequencies, higher power handling, lower modulation distortion, and lower intermodulation distortion, among other attendant benefits.
Among the first attempts at successfully implementing these specialized transducers were two-way coaxial loudspeakers wherein the high frequency transducer (tweeter) was centrally mounted with respect to the larger low frequency transducer (woofer), and hence shared a common axis. These loudspeakers were quite common and they also maintained the point source attribute previously mentioned. In most larger full range systems however (e.g., those manufactured and/or sold by Altec Lancing), this arrangement, owing mainly to a lack of space for a wider range tweeter, lead manufacturers to design and manufacture two-way systems wherein the tweeter was non-coincident with respect to the woofer. The tweeter was thus generally mounted above and in close proximity to the woofer. Although the woofer and tweeter were now sharing the sound spectrum equally, octave wise, the net result was a diminution of the point source effect.
As used herein, the term “coincident” means that the adjacent bandwidth transducers radiate from the exact same point in space and time, e.g., a 1 inch dome tweeter mounted atop a woofer pole piece. By contrast, a horn tweeter in which the tweeter voice coil may be positioned some distance behind the woofer voice coil is non-coincident, but coaxial because the horn tweeter voice coil has a “horizontal” displacement with respect to the woofer voice coil. If this same horn tweeter were instead mounted flush with the front baffle/woofer, it would now have a “vertical” displacement regardless of whether it is above, below, to the left of or to the right of the woofer.
As used herein, the term “non-coincident” therefore means an arrangement in which the relative displacement between/among the transducers is vertically offset, horizontally offset, or both.
In the course of time, it became known that the proximity of the woofer to the tweeter in non-coincident systems becomes critical if it was desired that the off-axis dispersion pattern (usually vertical) at the crossover region remains smooth. The requirement is that they be separated (center-to-center) by no more than a wavelength at the crossover frequency. For example, two transducers crossed over at 3 kHz should be no more than about 4.5″ apart, and two at 500 Hz should be no more than about 27.1″ apart. The inference is that this separation is especially critical at higher crossover frequencies.
In order to make further gains in the afore-mentioned criteria, especially in medium to low efficiency systems where the moving mass was generally higher, transducers were further specialized so that three-way systems were eventually made. The same criteria for crossover networks were utilized as before except that a midrange transducer generally required a band pass filter that restricted both its low and high frequencies. The three transducers in this case were generally arranged in a vertical, geometric configuration with the woofer near the bottom of the loudspeaker cabinet, followed by the midrange transducer and the tweeter near the top of the cabinet. The arrangement of these three transducers was an even greater departure from the point source effect than the two-way systems. Another configuration of three transducers was a triangular arrangement which arguably enhanced the point source effect.
Four-way and five-way non-coincident loudspeaker systems have also been implemented (hereinafter loudspeaker systems with four or more transducers will be referred to as “multi-way”).
Three-way and multi-way vertical alignments, with transducers generally arranged in sequential size order (with the largest transducer at the bottom and the smallest transducer at the top or vice versa) are quite common because they exhibit a generally smooth horizontal (left to right) polar pattern which is considered important in order to obtain a stable stereo image. However, these alignments are significantly incoherent, both on and off axis, as explained below.
Multi-way loudspeaker systems may have been constructed in consideration of the above recommendations pertaining to the criteria for choosing crossover points and the proximity of any two adjacent bandwidth non-coincident transducers. Their implementation was also influenced by the need to maintain at least a three-octave spread between crossover frequencies in a three-way system so as to minimize interference patterns between the transducers.
However, in prior art three-way and multi-way loudspeaker systems, there does not appear to be any recommendation or scientific method that pertains to proportioning adjacent bandwidth transducers with respect to relative size (radiation resistances). Radiation resistance of a transducer determines the power output and is a function of the frequency propagated, the method of coupling, and the size of the transducer. The radiation resistance of an unbaffled transducer in free air increases from a very low value to a value of approximately 42 acoustic ohms per square centimeter, which is the acoustic impedance of air. Maximum power will be transmitted to the air when the transducer approaches this impedance because the generator impedance will equal the load impedance. In the case of a circular diaphragm, this occurs when the diameter is equal to or slightly less than the wavelength being propagated. As the frequency increases and the wavelength is increasingly smaller than the diameter, the output power remains constant. However, in this event, the polar pattern becomes narrower and the higher frequencies are “beamed”.
In the frequency range where the wavelengths are larger than the diameter of the diaphragm, a baffle or enclosure is required to prevent the front wave from canceling the rear wave, thus providing it with a proper load into which it operates to produce acoustic power (usually rated in acoustic watts).
If the wavelength or frequency is left unchanged, and the diameter of the transducer decreases, the radiation resistance per unit area drops, as does the power for that frequency. If the transducer size remained the same, and instead the wavelength increased (correlating to a reduction in the frequency), the ratio of the diameter of the transducer to the wavelength would also decrease, and again there would be a drop in the radiation resistance of the transducer and consequently less power would be radiated.
This explains a common phenomenon in the low frequencies: for a given low frequency, the smaller the transducer, the less the power output, and for a given size transducer, the low frequency power output will drop as the frequency is decreased. This phenomenon, however, is perhaps less noticed in the rest of the audible frequency range. It is a parameter which is almost entirely overlooked in that it is quite common to find two-way loudspeaker systems crossed over and which have adjacent diaphragm area ratios in the neighborhood of about 20:1 (e.g., a 178 mm woofer and a 28 mm tweeter) and although displaying smooth frequency responses, the power response (which is the power output, in acoustic watts, at all frequencies-on and off axis, usually into 180 degrees or 2π radians) is poor. Although power response is an absolute quantity, this is also a result of the large disparity in the relative radiation resistances of the two drivers (discussed below). Although moving coil (dynamic) transducers generally exhibit a somewhat variable mass characteristic (if the diaphragm is not overly rigid), they are still mass-controlled devices and respond accordingly.
An ideal loudspeaker or loudspeaker system would therefore propagate its power in radiation resistances which are independent of frequency (as in a continuum).
In addition to a lack of a recommendation regarding radiation resistances, in prior art three-way and multi-way loudspeaker systems, there also does not appear to be any recommendation or scientific method that pertains either to proportioning adjacent bandwidth transducers with respect to voice coil size and moving mass. Moreover, there does not appear to be any disclosure of geometrically configuring a three-way or multi-way loudspeaker system so that their combined outputs may coalesce at a defined point, or along a defined line in space, thereby unifying the resultant sound field into a virtual point source.
Without such a recommendation based on a scientific methodology, these design parameters have been left, to a greater or lesser extent, to the whim of the system designer, and therefore still reside in the area known as “black art”. As a result, the vast majority of three-way and multi-way non-coincident loudspeaker systems, regardless of type, either have individual transducers incorrectly proportioned to one another, and thus do not seamlessly “blend” with each other (i.e., there are discernible transitions between adjacent bandwidth transducers resulting from an adverse interrelationship of diaphragm diameters, voice coil diameters, moving masses, efficiencies, overlapping bandwidths, crossover type and slope, etc.) and/or are incorrectly arranged geometrically and therefore do not behave as a virtual point source.
Disclosed herein is a multi-way loudspeaker that achieves both seamless “blending” and virtual point source behavior by relating the transducers to each other, and each transducer to the assembly of transducers.
A discussion will now be provided of various loudspeaker systems.
A first type is a multi-way planar electrostatic loudspeaker and a multi-way planar magnetic loudspeaker. Although these loudspeakers differ in the type of driving force utilized, they share the characteristic of being equally driven over most or all of their area (unlike a centrally-driven voice coil of a moving coil dynamic loudspeaker, or a dome type which is usually driven at its periphery). Since they are generally limited in their diaphragm excursion ability, this necessitates larger diaphragm areas for adequate sound pressure levels. As a result, these diaphragms are generally designed in the shape of long and narrow rectangles which are vertically oriented so that they have a wider horizontal than vertical dispersion, and as such, are not considered point sources, but plane wave or line sources.
Typically, the design criteria used for such loudspeakers is to make the diaphragm length, including the wall or floor reflection, larger than λ/3 (wavelength/three) for the lowest frequency of interest, and small compared to λ/3 for the highest.
Line sources typically display a “time smear” as the path lengths of the output waveform at primarily middle and high frequencies differ greatly from various parts of the diaphragm to the listener (at any reasonable distance). In the case of three-way or multi-way systems, the configuration itself generally adds incoherence to the “time smear”.
In contrast to such prior art loudspeakers, disclosed below is a multi-way planar loudspeaker system which propagates a unified sound field along a single axis, and therefore behaves as a quasi virtual point source (“quasi” because since planar diaphragms are essentially driven equally over their entire area, the sound source does not collapse towards the center of the diaphragm as the frequency increases as in most cone-type moving coil transducers, and therefore are not intrinsically point sources). Additionally, dependent on size and complexity, the power output of a multi-way planar system in accordance with the invention may be virtually independent of frequency.
A second type of sound transducing system is a full frequency range single diaphragm condenser/electret condenser microphone. A microphone is in essence is a loudspeaker in reverse, i.e., the sound waves impinging on the diaphragm generates a voltage which is amplified, and then is either used for recording purposes or sent to a loudspeaker for sound reproduction or reinforcement purposes. Although there have been two-way microphones designed and manufactured, the overwhelming type currently known to be in existence are of the single diaphragm type. Of these, the four main types are the dynamic (moving coil), condenser (electrostatic), electret condenser (permanently charged electrostatic) and the ribbon (a type of dynamic).
In the condenser field, diaphragm sizes of ½″ and 1″ (circular) are the most common with the former generally considered the most accurate overall (of any type) both in frequency and transient response. The recording industry values other sizes and types for their unique “colorations” which, when utilized properly, enhance certain instruments and/or vocals. The larger 1″ diaphragm condenser microphone, for example, is often preferred for vocals as it renders a “larger than life” sound when placed close to a vocalist.
Regardless of type, most if not all known microphones use diaphragms having a symmetrical shape, whether it is the radial symmetry of a circle or the bilateral symmetry of a rectangle. As such, these diaphragms, regardless of driving force, will tend to favor various bandwidths of frequencies solely in view of their shape.
Although the present approach is to use a microphone with the opposite characteristics of the instrument or vocal being recorded for a complementary result, the invention disclosed below, in view of the presence of a diaphragm having an asymmetrical shape, serves greatly to mitigate this requirement, as this shape does not favor any frequency or band of frequencies, but instead exhibits a virtually uninterrupted and seamless continuum of radiation resistances (in reverse) of all audible frequencies (commensurate with its size and complexity). An additional advantage of this asymmetrical shape characteristic is that the microphone may be designed specifically as left and right channel configurations. The embodiments of the invention disclosed below are particularly suited to condenser and electret condenser types, but are also applicable to single element, full range electrostatic speakers as discussed below. It is also applicable to headphones, hearing aids and other similar devices.
A third type of sound transducing system is a full range, single diaphragm electrostatic loudspeaker. Since electrostatic loudspeakers are resistance-controlled devices (as opposed to mass-controlled), a single diaphragm has been utilized for a full range high fidelity loudspeaker, albeit with lower efficiency being one of the tradeoffs. These full range, single diaphragm electrostatic loudspeakers have generally utilized large and generally curved symmetrical diaphragms (for greater output and lateral dispersion).
In the invention, using an asymmetrical, non-coincident diaphragm, a continuum of radiation resistances is obtained which is virtually independent of frequency, and a virtual point source characteristic is also obtained, as opposed to that of a line source as described above. Elimination of the crossover(s) and their attendant phase shifts is an additional benefit.
With respect to specific prior art, U.S. Pat. No. 3,645,355 to Long describes a loudspeaker system having a predetermined center-to-center spacing of two speakers. Reference is made to low frequency drivers only and the preferred spacing is from 6″ to 9″ apart for an allegedly improved high frequency roll off characteristic. No geometry is provided for the high frequency driver(s).
U.S. Pat. No. 3,824,343 to Dahlquist describes a multiple driver dynamic loudspeaker including an array of transducers which is not planar, but three-dimensional. The rise time characteristic is adjusted for adjacent pairs of transducers which are moved forwardly or rearwardly (horizontally) relative to each other so as to achieve a desired result.
U.S. Pat. No. 4,031,318 to Pitre describes a high fidelity loudspeaker system including multiple disparate drivers covering the same bandwidth of frequencies and arranged along three sides of a loudspeaker cabinet. The loudspeaker system utilizes a crossover network which is juxtaposed with a separately enclosed multiple driver mid frequency array which overlaps the output of the two-way, and utilizes only a high pass filter.
U.S. Pat. No. 4,119,799 to Merlino describes a loudspeaker cabinet system including two identical low frequency drivers spaced apart from one another such that a center to center distance is the piston diameter of the smaller driver times Pi (π). The reference is to low frequency drivers only and states that the high frequency driver or array may be positioned “thereabout”. No overall geometry is given.
U.S. Pat. No. 4,730,694 to Albarino describes a high fidelity loudspeaker enclosure including multiple drivers in various configurations. No rationale is provided for the different configurations of drivers.
U.S. Pat. No. 4,885,782 to Eberbach describes loudspeaker driver configurations in which the symmetry in placement of the drivers is substantially more important than the distances between drivers. No precise geometry is provided for the location of the drivers. One array of drivers shows an angle of “turn” (centered on the tweeter) in excess of one hundred and fifty degrees (150°).
U.S. Pat. No. 5,164,549 to Wolf describes a sonic wave generator including concave baffles which are preferably dimensioned in a specific relationship relative to one another. For example, mention is made of a ratio of an upper to an immediately lower baffle of 1:0.66. No mention is made of the proportion of the transducers therein or the distances between them.
U.S. Pat. No. 5,430,260 to Koura et al. describes a speaker system utilizing four woofers and one tweeter. The patent pertains to a bilaterally symmetric configuration of woofers around a centrally mounted tweeter for controlled vertical dispersion.
The prior art does not disclose any specific, scientific methods for arranging transducers or drivers in a multi-way loudspeaker system.